Thermodynamic fluids



Filed July 2, 1968 TURBINE CONDENSER BOILER PUMP FIGUREIL m B 0 DM E E H NE 0 ..5b A H R N R PU T A U UO 5.: N H T D TP 05 C B M 5V TR II N L P 3 O w 1 n U N am E L 5 o A-U 0. O O 0 0 0 O m m M m m a e 4 2 o FIGURE 2 INVENTOR MALCOLM Mc EWEN v BY i? ATTORNEY United States Patent 0 3,516,248 THERMODYNAMIC FLUIDS Malcolm McEwen, Glendale, Mo., assignor to Monsanto Company, St. Louis, Mo., a corporation of Delaware Filed July 2, 1968, Ser. No. 742,093 Int. Cl. F01k 25/00; C09k 3/18 US. Cl. 60-36 24 Claims ABSTRACT OF THE DISCLOSURE The present invention relates to Rankine cycle working fluids. More specifically, it relates to certain organic compounds which have been found outstanding for use in Rankine cycle equipment.

In a typical Rankine cycle power system, a suitable working fluid is passed in heat exchange relationship with a source of heat of suflicient intensity to vaporize the fluid. Thereupon, the energy of the expanding vapors is utilized to produce work by passing the vapors through a turbine or other work-producing device. It is common to condense the vapors and pump the condensed liquid back in heat exchange relationship with the heat source to complete the cycle.

Water or steam has been the most commercially utilized working fluid for Rankine cycle systems. The disadvantages of water or steam as a working fluid include those of high boiling point, high critical pressure and low density, all of which factors limit the power obtainable and result in the need for comparatively large and expensive apparatus. Furthermore, steam is defincient in entropy relationships along its saturated vapor line, showing an increasing entropy with decreasing pressure, and excessive condensation upon isentropic expansion. Accordingly, the use of a steam cycle normally requires superheating and resuperheating to prevent condensation of liquid in the turbine or other work-producing device, which liquid can cause erosion of metal and loss of efficiency. Unless provision is made for superheating, therefore, the expanded vapor may have an excessively high moisture content.

A variety of fluids have been evaluated in the past as working fluids for power cycles. Mercury is an example of such fluid; however, the use of mercury in the Rankine cycle entails the disadvantage of high cost, and also that of exhibiting unfavorable entropy changes along the saturated vapor line. Furthermore, mercury is toxic and forms amalgams with certain metals.

The prior art has disclosed only a limited number of organic compounds as suitable working fluids for Rankine cycle use. Typical examples of prior art organic working fluids are biphenyl and the aliphatic fluorocarbons.

As efiorts are made to reduce costs, improve operating efficiencies and reduce water consumption, the recovery and conversion to shaft power of waste heat from industrial process plants, chemical plants, oil refineries and prime movers such as gas turbines has become of considerable interest. Waste heat recovery systems based on the Rankine cycle, therefore, have many present day advantages and the thermal requirements for such systems have introduced a need for improved working fluids.

In the present invention it has been discovered that certain organic compounds can be advantageously employed as working fluids for Rankine cycle systems. These fluids are characterized by having on a Mollier diagram a maximum entropy change between two established state points.

It is an object of the present invention, therefore, to provide a group of outstanding working fluids for Rankine cycle power systems.

3,516,248 Patented June 23, 1970 Another object of the present invention is to provide an improved method for operating Rankine cycle equipment through incorporation of these outstanding working fluids.

Still another object of the present invention is to provide an improved Rankine cycle power system which utilizes certain outstanding chemical compounds as the working fluid.

Yet another object of the present invention is to provide a Rankine cycle power system utilizing working fluids which produce higher efficiencies and permit lower system costs and operating costs than many conventional fluids.

Still another object of the present invention is to provide a Rankine cycle power system capable of utilizing low temperature waste heat as the heat source.

Still another object of the present invention is to provide a Rankine cycle power system in which the working fluid, in certain cases, can be used to lubricate the bearings and certain other working parts of the associated machinery.

Other objects, aspects and advantages of this invention will become apparent from a consideration of the accompanying disclosure, drawings, and claims.

In the drawing:

FIG. 1 is a diagram of a basic Rankine cycle.

FIG. 2 is a Mollier diagram of 1,4-diazine, an organic compound within the scope of the present invention.

Broadly stated, the present invention discloses certain sulfur-free, non-halogenated organic compounds having an entropy change of not more than 0.10 B.t.u. per lb. per R. between the atmospheric pressure vapor point and the maximum enthalpy point, both on the saturation curve of the Mollier diagram.

Because of the relatively low entropy change of the fluids taught by the present invention, it becomes possible to optimize the efliciency of Rankine cycle systems without the need for auxiliary equipment such as regenerators and superheaters. The outstanding working fluids of the present invention offer substantial advantages, therefore, in the design of compact Rankine cycle equipment.

Referring to FIG. 1 of the drawing, the essential components of a basic Rankine cycle system are illustrated diagrammatically. In such a system the working fluid is pumped into heat exchange relationship with a heat source such as a boiler. The heat from the heat source vaporizes the working fiuid and the vapors are then passed through a work-producing device, such as a turbine, in order to convert the energy of the expanding vapors into useful mechanical energy. The working fluid vapor is ideally expanded at constant entropy, and a portion of the available energy is converted into useful work. The function of converting heat energy into mechanical energy has been fulfilled at this point. In order to reuse the working fluid, however, the cycle is completed by further cooling the vapor which has passed through the work producing device, liquefying the vapor at constant pressure such as in a condenser, and then pumping the condensed liquid back to the boiler or heat source for reuse.

A number of variations may be made on the basic Rankine cycle system of FIG. 1, all of which are well known in the art. Among these cycle variations are the regenerative cycle, the binary cycle, the saturated cycle, the superheated cycle, and the supercritical cycle. The outstanding working fluids of the present invention are adaptable to these and other Rankine-type cycles.

Application of the working fluids of the present invention in Rankine-type systems and cycles may be made for the purpose of utilizing heat energy, and particularly waste heat energy, which is available from a variety of sources which have been previously utilized as sources of heat energy for power cycles. Furthermore, because of their high thermodynamic or Rankine cycle eificiency, the fluids of the present invention are particularly advantageous for recovering and converting heat energy to mechanical energy from relatively low level heat sources. Typical heat sources which may be taken advantage of include, but are not limited to, the following:

hot stack gases from industrial processes, exothermic heat from various chemical reactions, natural thermal Wells, and exhaust from gas turbines and other internal combustion machines.

Some specific applications which may be cited as exemplary for use of the working fluids of the present invention include: the utilization of energy from turbine exhaust gases to drive an auxiliary turbine; the recovery of heat in chemical synthesis plants and the conversion thereof to mechanical energy to operate auxiliary equipment; the recovery of Waste heat from pumps, fluid motors, turbines and expanders, which is converted to power for driving pumps, compressors, blowers, electrical generators and the like.

Referring now to FIG. 2 of the drawing, a Mollier diagram for 1,4-diazine is presented. At the intersection of the atmospheric pressure line and the saturated vapor line, a point designated by the letter A is established. The maximum saturation enthalpy point on the Mollier diagram is designated by the letter B. The change in entropy between point A and point B on the Mollier diagram of FIG. 2 represents the basis on which the fluids of the present invention are defined and limited. With reference to the relative entropy scale on the abscissa of the diagram, the approximate entropy at point A is 0.035 B.t.u. per pound per R. The entropy at point B is approximately 0.060 B.t.u. per pound per R. The entropy change from A to B, therefore, is an increase of approximately 0.025 B.t.u. per pound per R.

With continued reference to FIG. 2, it is the magnitude of the entropy change which has significance herein rather than the direction of the entropy change. In the case of 1,4-diazine, the entropy change from point A to point B On the Mollier diagram is a positive quantity. For certain other compounds within the present invention, the entropy change from point A to point B may be a negative quantity. Superior results are obtained for Rankine cycle use so long as the entropy change between points A and B does not exceed 0.10 B.t.u. per pound per R., regardless of whether the entropy at point B is greater or less than that at point A.

By way of example and not in a limiting sense, the following organic compounds are representative of the propionitrile, terephthalonitrile, acrylonitrile, acetamide, pyridine, ethyl chloride, pyrrole, pyrazole, imidazole, cyclopentadiene, acetic anhydride, acetic acid, propane, dimethylamine, nitromethane, ethylene glycol, tertbutanol, dimethyl and diethyl ethers, dimethoxy methane, dimethyl propanediol, phenol, toluene, aniline, pyrrolidine, ethanol, cycloproparie, trimethoxy methane, tetramethoxy methane, benzoic acid, succinic anhydride, maleic anhydride, acetonitrile, caprolactone, cyclohexane, n-hexane, piperidine, trimethyl triazine, benzofuran, tetramethyl urea, morpholine, phenyl hydrazine, tetramethylethylene, nitrobenzene, acetophenone, ethylene glycol diacetate and dimethylacetamide.

The organic working fluids 'within the scope of the present invention include aliphatic, carbocyclic, substituted carbocyclic, heterocyclic and fused ring organic compounds. The substituted carbocyclic compounds can include, for example, alkyl, aryl, aralkyl, alkoxy, acyloxy, nitro, nitiile, amino, hydroxy, alkenyl and aryloxy substituents. In the case of the heterocyclic compounds, one ore more of the hetero atoms can be nitrogen, oxygen, boron, phosphorus or silicon, or a combination of two or more of said atoms.

Illustrative of the various radicals which can appear as substituents in the compounds of the present invention are alkyl radicals, e.g., methyl, ethyl, propyl, butyl, octyl; aryl radicals, e.g., phenyl, naphthyl, tolyl, xylyl, etc.; aralkyl radicals, e.g., benzyl, phenylethyl, etc.; alkenyl radicals, e.g., vinyl, allyl, etc.; alkoxy radicals, e.g., methoxy, ethoxy, propoxy, butoxy, etc.; acyloxy radicals, e.g., acetoxy, butoxy, etc.; aryloxy radicals, e.g., phenoxy, naphthoxy, aryloxy-substituted phenoxy, alkoxy-substituted phenoxy, alkyl-substituted phenoxy, aryl-substituted phenoxy, etc. Other exemplary substitutents are nitrile, nitro, hydroxy, amino, or multiples or combinations of these or any of the aforementioned substituents.

It has been found that halogenated organic compounds, such as the aliphatic fluorocarbons, often lack the degree of thermal stability required of a superior Rankine cycle working fluid. Sulfur-containing organic compounds tend to be corrosive to ordinary materials of construction found in Rankine cycle equipment. Fluids with low thermal stability and high corrosion rates are not desirable as Rankine cycle working fluids, notwithstanding other advantangeous properties which they might possess, such as high Rankine cycle efliciency.

The organic Working fluids taught by the present invention embrace a large number of chemical compounds. For illustrative purposes, some physical properties of several representative compounds are presented in Table I below:

TABLE I.PROPERTIES OF ORGANIC WORKING FLUIDS Entropy change from Points A to B as in I Melting Bolling Molecular Critical Critical Critic per 1b.

Compound point F.) point F.) weight temp. F.) pressure (p.s.i.a.) volume (ftfi/lb.) per "R;

Pyridine 44 239 79 657 817 0. 05134 0. 036 237 81 660 1078 0. 04672 0. 021

sulfur-free, non-halogenated fluids of the present invention having an entropy change of not more than 0.10 B.t.u. per pound per R. from the condition represented by point A to the condition represented by point B on the Mollier diagram: 1,3,5-triazine, 1,4-diazine, furan, cyclopentane, propanol, isopropanol, isobutane, isobutylene, dimethyl acetylene, glycerol, trimethyl amine, ethylenediamine, acetone, methyl acetate, malononitrile,

Rankine cycle efliciencies were computed for some represented working fluids of the present invention for comparison with conventional fluids such as Water and fluorocarbon refrigerant 12. These data are presented in Table II and the efliciencies were calculated on expansion from 500 F. to 100 F., assuming a constant turbine efliciency of percent.

TABLE II Compound: Cycle efiiciency, percent Flourocarbon refrigerant 12 15.7 Water 30.5 Pyridine 26.1 1,3,5-triazine 26.7 Toluene 24.1 1,4-diazine 26.2 Cyclopentane 22.6 Cyclohexane 22.1

From the data of Table 'II it is seen that the six exemplary organic fluids of the present invention have substantially higher efliciencies than fluorocarbon refrigerant 12. Although water has a somewhat higher efficiency than the cited organic fluids, this gain is more than ofiset by the disadvantages caused by the deficient entropy characteristics of steam.

In addition to those sulfur-free, non-halogenated organic compounds meeting the entropy requirements established herein, it is to be understood that the present invention also includes mixtures or blends of one or more of said compounds wherein the entropy change of the mixture does not exceed 0.10 B.t.u. per pound per R. at the conditions established. Also included within the present scope as Rankine cycle working fluids are mixtures wherein one or more of the included compounds has entropy characteristics outside the limits established herein but wherein entropy change of the mixture does not exceed 0.10 B.t.u. per pound per R. It is to be further understood that certain additives may be added in minor amounts to the fluids of the present invention in order to obtain desired physical effects.

From the above description of the present invention, it will be apparent that the advantageous properties of the working fluids disclosed herein make the fluids adaptable to a variety of well known power recovery systems and thermodynamic cycles. It will be equally apparent that Other variations, cycles and applications may be readily devised and employed by those skilled in the art, which may also be used to take advantage of the outstanding working fluids of the present invention. By way of further example, a regenerative system may be employed with a supercritical cycle and binary systems may be employed with either saturated or superheated cycles, alone or in combination with a regeneratlve system.

While this invention has been described with respect to certain specific examples, it is not so limited, and it is to be understood that variations and modifications thereof may be made without departing from the spirit of the following claims:

The embodiments of this invention in which an exclusive property or privilege is claimed are defined as follows:

1. A method of converting heat energy to mechanical energy which comprises the steps of:

(a) vaporizing a fluid by passing the same in heat exchange relationship with a heat source, said fluid comprising a sulfur-free, non-halogenated organic compound characterized by an entropy change not greater than about 0.10 B.t.u. per pound per R. between the entropy at the maximum saturation enthalpy point on a Mollier diagram and the entropy of the saturated vapor at atmospheric pressure; and

(b) utilizing the energy of the vaporized fluid to perform work.

2. A method of claim 1 wherein the vaporized fluid is condensed and recycled to pass in heat exchange relationship with the heat source.

3. A method of claim 1 wherein the vaporized fluid is expanded through a turbomachine.

4. A method of claim 1 wherein the compound is aliphatic.

5. A method of claim 1 wherein the compound is carbocyclic.

6. A method of claim 1 wherein the compound is substituted carbocyclic in which the substituents are selected from the group consisting of lower alkyl, aryl, aralkyl, alkoxy, acyloxy, alkenyl, aryloxy, nitro, nitrile, amino and hydroxy.

7. A method of claim 1 wherein the compound is heterocyclic in which the hetero atoms are selected from the group consisting of nitrogen, oxygen, boron, phosphorus and silicon, or a combination of two or more of said atoms.

8. A method of claim 1 wherein the compound has from about 2 to about 6 fused rings.

9. A method of claim 1 wherein the compound is pyridine.

10. A method of claim 1 wherein the compound is toluene.

11. A method of claim 1 wherein the compound is 1,4- diazine.

12. A method of claim 1 wherein the compound is 1,3,5-triazine.

13. A method of converting heat energy to mechanical energy which comprises the steps of:

(a) vaporizing a fluid by passing the same in heat exchange relationship with a heat source, said fluid comprising a mixture of sulfur-free, non-halogenated organic compounds, each of said compounds characterized by an entropy change not greater than about 0.10 B.t.u. per pound per R. between the entropy at the maximum saturation enthalpy point on a Mollier diagram and the entropy of the saturated vapor at atmospheric pressure; and

(b) utilizing the energy of the vaporized fluid to perform work.

14. A method of converting heat energy to mechanical energy which comprises the steps of:

(a) vaporizing a fluid by passing the same in heat exchange relationship with a heat source, said fluid comprising a composition containing a major amount of sulfur-free, non-halogenated organic compound characterized by an entropy change not greater than about 0.10 B.t.u. per pound per R between the entropy at the maximum saturation enthalpy point on a Mollier diagram and the entropy of the saturated vapor at atmospheric pressure, said composition being characterized by said entropy change; and

(b) utilizing the energy of the vaporized fluid to perform work.

15. In a power generating system in which a working fluid is vaporized, expanded through a machine to produce work, and cooled to a liquid state, the improvement which comprises using as said working fluid a sulfur-free, non-halogenated organic compound characterized by an entropy change not greater than 0.10 B.t.u. per pound per R between the entropy at the maximum saturation enthalpy point on a Mollier diagram and the entropy of the saturated vapor at atmospheric pressure.

16. A system of claim 15 wherein the compound is aliphatic.

17. A system of claim 15 wherein the compound is carbocyclic.

18. A system of claim 15 wherein the compound is substituted carbocyclic in which the substituents are selected from the group consisting of lower alkyl, aryl, aralkyl, alkoxy, acyloxy, alkenyl, aryloxy, nitro, nitrile, amino and hydroxy.

19. A system of claim 15 wherein the compound is heterocyclic in which the 'hetero atoms are selected from the group consisting of nitrogen, oxygen, boron, phosphorus and silicon, or a combination of two or more said atoms.

20. A system of claim 15 wherein the compound has from about 2 to about 6 fused rings.

21. A system of claim 15 wherein the compound is pyridine.

7 8 22. A system of claim 15 wherein the compound is 3,234,734 2/1966 Buss et a1. 60--36 toluene. 3,282,048 11/1966 Murphy et a1. -60-36 23. A system of claim 15 wherein the compound is 3,407,232 10/1968 Mitsch 25267 XR 1,4-diazine. 3,423,469 1/1969 Hatton et a1 25267 XR 24. A system of claim 15 wherein the compound is 5 1,3,5-triaziue. FOREIGN PATENTS R d 280,926 9/ 1928 Great 'Britain. defences 285,374 7/1928 Great Britain. UNITED STATES PATENTS 294,243 6/1929 Great Britain. 1,818,117 8/1931 Davenport 25267 10 389,789 6/1931 GreatBmam- Z321 g i jg MARTIN P. SCHWADRON, Primary Examiner 2,301,404 11/1942 Holmes 60-36 R. R. BUNEVICH, Assistant Examiner 3,040,528 6/1962 Tabor et a1. 6036 154,928 11/1964 Harmens 60-36 XR 15 U.S.C1.X.R.

3,189,621 6/1965 Harnik 25267 XR 25267 

